This set of routines use the parent nexus driver to perform conversions in page size units.
ddi_btop() converts the given number of bytes to the number of memory pages that it corresponds to, rounding down in the case that the byte count is not a page multiple.
ddi_btopr() converts the given number of bytes to the number of memory pages that it corresponds to, rounding up in the case that the byte count is not a page multiple.
ddi_ptob() converts the given number of pages to the number of bytes that it corresponds to.
Because bus nexus may possess their own hardware address translation facilities, these routines should be used in preference to the corresponding DDI/DKI routines btop(9F), btopr(9F),
and ptob(9F), which only deal in terms of the pagesize of the main system MMU.
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